Wednesday, May 24, 2017

Medical power supplies meeting IEC 60601-1-2 4th edition voltage dips and interruptions

Customers call TDK-Lambda wanting their medical product to meet the strict IEC 60601-1-2:2015 4th edition immunity standard, and ask us if our medically certified power supplies fully comply.  In particular, concerns are raised about meeting the section dealing with voltage dips and short interruptions to the AC supply.
IEC 60601-1-2 is derived from the IEC 61000-4 standard, which covers Electromagnetic compatibility (EMC).  The testing and measurement methods are very similar, but some of the test levels for dips and interruptions in the section based on IEC 61000-4-11 are much tougher.  The interruption test of removing the AC supply for 5 seconds, without the loss of the output, is almost impossible without a custom solution with some form of battery back-up.  One may well question why are standard medical power-supplies being sold if they do not meet that standard.
Firstly, power supplies are not classified as medical devices, it is the customer’s product or system that is the medical device.
Secondly the term “essential performance” used in the standard has to be examined.  In the 3rd edition of IEC 60601-1 it is defined as “the performance necessary to achieve freedom from unacceptable risk”.  To clarify, the designer/manufacturer has to determine if a loss of performance or functionality of their medical device product or system will result in an acceptable risk or an unacceptable risk.  That risk is the potential to harm a patient, operator or the environment.  Analysis must be made of the probability or the frequency of an event happening compared to the severity of that event.
Let’s give a simple example.  Diabetics check their blood glucose level on a regular basis and most use a handheld battery-operated meter that accepts disposable test strips.  If that meter was to stop working, say due to a faulty display, it would be classified as an acceptable risk.  Replacement meters are readily available from supermarkets and pharmacies and a short delay in testing would not normally cause harm.  An unacceptable risk would be if the internal sensor measuring the blood glucose level was to produce incorrect readings and the diabetic administered too much or too little insulin.
Power supplies, although not classified as medical devices, can have an impact on the IEC 60601-1-2 immunity performance of the device they are powering.  For the voltage dips and interruptions section of the standard, there are five tests performed.  Table 1 below shows the input voltage dip and the duration.  100Vac input and 50Hz conditions are shown as they could represent the worst case.
Test results are judged against four performance criteria levels:
Performance Criteria A – ‘Performance within specification limits’
This is the best result.  A very slight drop in output of a few milli-volts (within the regulation limits) should not cause the end device to malfunction.
Performance Criteria B – ‘Temporary degradation which is self-recoverable’
Criteria B is usually acceptable in the majority of cases.
Performance Criteria C – ‘Temporary degradation which requires operator intervention’
This would be classified as unacceptable from a user point of view, without even considering a risk analysis.  If the AC power was interrupted and the power supply had to be reset by a patient or operator, it would be much too inconvenient.
Performance Criteria D – ‘Loss of function which is not recoverable’
Criteria D is really a “fail” test result.  If a power supply is damaged and needs replacing after the test, it is very unlikely that a product with this performance level would be placed on the market.
AC Input Voltage
Actual Voltage Dip for 100Vac nominal
Voltage Dip by AC Input Cycle
Voltage Dip Time Period for 50Hz
Suggested Performance Criteria Level
Dip down to 0%
0.5 of a cycle
Dip down to 0%
1 cycle
Dip down to 40%
10/12 cycles
Dip down to 70%
25/30 cycles
Dip down to 0%
250/300 cycles
5000ms (5s)
Table 1: Test Levels
Referring to Table 1, most power supplies will pass the first two tests with a Performance Criteria level A with some output derating to increase the hold-up time.
The third and fourth tests requires the power supply to continue to operate for 200ms when the input drops to 40% of nominal or for 500ms at 70% of nominal.  Criteria A could be achieved by having the power supply’s low voltage input protection circuitry modified to allow the power supply to operate at the lower input voltage for a short time.  As the AC input current will be higher, it is best to ensure that the power supply is not operated at full load.  As hold-up time is related to the actual output power drawn, operating the power supply at 50% load will result in a significant “ride through” capability during the interruption.
The fifth test of a 5 second interruption to the AC supply is usually met with the installation of battery back-up or a UPS (Uninterruptible Power Supply).  Adding sufficiently large energy storage inside the power supply would result in a significant increase in size.
In summary, the medical device designer/manufacturer must decide which performance criteria is needed, based on their risk analysis to meet IEC 60601-1.  Unless continuous performance is critical, most manufacturers will opt for the criteria in Table 1.

Tuesday, January 17, 2017

How will the Power Supply Industry be affected by EN 55032 replacing EN 55022?

In 2014 the Hazard-Based Safety Engineering (HBSE) standard IEC 62368-1 was announced combining the Information Technology Equipment (ITE) standard EN 60950-1 and the audio, video and similar electronic apparatus safety standard EN 60065.  This step was taken as there was no longer a clear definition between ITE and multimedia equipment with advent of internet connected TVs, smartphones and other home entertainment products.
Now the EMC standards are also being combined and as of March 5th, 2017, EN 55022, EN 55013 and EN 55103 will be replaced by one unified emission requirements standard called EN 55032.  The current “Electromagnetic compatibility of multimedia equipment” was first published in May 2012 as EN 55032:2012+AC 2013, will be withdrawn on May 5th 2018.  EN 55032:2015+AC:2016, which was announced May 2015 and published February 2016, has already superseded the 2012 standard.
The three standards that are being replaced by EN 55032 are:
EN 55022: Information Technology Equipment, Radio disturbance characteristics. Limits and methods of test.
EN 55013: Sound and Television Broadcast Receivers and Associated Equipment.
EN 55103: Audio, Video and Entertainment Lighting Equipment for Professional Use.
Fortunately for those in the power supply industry serving the ITE market who have relied on EN 55022 as their core standard for many years, there are no changes to the test requirements.  The multimedia equipment (MME) makers will have additional test requirements to interface ports, port type and emissions from cabling.  The individuals that prepare and sign their company’s CE Declaration of Conformity will be kept busy updating their forms though!
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